The present invention concerns light totalizers for cameras provided with shutter-diaphragms. A shutter-diaphragm is a structure which defines the exposure aperture of the camera and is operative, during the course of an exposure, for progressively opening up the exposure aperture from zero or a minimum aperture size through intermediate aperture sizes up to a maximum aperture size. After the elapse of an appropriate exposure interval, the exposure aperture of the camera is abruptly blocked against incoming light, in response to a terminate-exposure signal. Typically, the terminate-exposure signal is produced by a light integrator or totalizer. The light integrator or totalizer comprises a photosensitive element exposed to ambient scene light and operative for generating a light-dependent signal, plus also integrating or totalizing circuitry operative for integrating or totalizing the light-dependent signal to yield a totalized-light signal. When the totalized-light signal reaches a predetermined value, the terminate-exposure signal is generated. If the scene-light level is very low, the exposure will in general not be terminated until after the shutter-diaphragm has assumed its maximum aperture size, with a sizable fraction of the total duration of the exposure being performed at maximum exposure-aperture size. If the scene-light level is somewhat higher, a smaller fraction of the total duration of the exposure is performed at maximum exposure-aperture size. For scene-light levels which are not low, the terminate-exposure signal is generated before maximum exposure-aperture size is even reached by the shutter-diaphragm.
For light totalizers used with such shutter-diaphragms, the scene-light signal produced, being as it is an indication of instantaneous exposure light, must take into account the progressive increase in exposure-aperture size occurring during the initial part, or indeed the entirety, of the exposure. This can be done by modifying the light-dependent signal totalized by the light totalizer in such a way that its value be dependent not only upon the ambient scene light to which the light detector of the system is exposed but additionally dependent, in one or another way, upon the progressive increase in exposure-aperture size. For example, in the case of shutter-diaphragms of the type which implement a linear increase of exposure-aperture surface area up to the point where maximum exposure-aperture size is reached, a properly designed light-totalizing system should nominally totalize a scene-light signal whose instantaneous value increases in correspondence to increasing aperture size during the opening-up of the exposure aperture, and thereafter remains at a constant value, determined by the ambient scene-light level, for the subsequent portion, if any, of the exposure during which the exposure-aperture stays at maximum size.
This can be accomplished in various ways. For example, it is known to generate the light-dependent signal to be totalized using a first photosensitive detector during the opening-up of the exposure aperture, and a second photosensitive detector subsequent to the exposure aperture reaching its maximum surface area.
However, we have considered the use of an entirely different type of light totalizer which comprises a pulse generator, including a photosensitive element exposed to ambient scene light, operative for generating a pulse train whose pulse repetition frequency varies in dependence upon the scene-light level, with the totalizer stage of the system being a digital counter operative for counting pulses from the light-dependent pulse generator and producing a terminate-exposure signal at least in dependence upon the reaching of a predetermined count. In particular, we have considered how, with a light totalizer of this type, the progressive exposure-aperture size increase which occurs with a shutter-diaphragm can be taken into account.
If, during the course of the exposure, the exposure aperture's surface area is increasing as a linear function of elapsed time, a plot of exposure-aperture surface area versus elapsed time would be an inclined line of nominally constant slope. Accordingly, from the nature of the problem presented by such shutter-diaphragms, a natural approach would be to make the value of the scene-light signal (i.e., the pulse repetition frequency of the light-dependent pulse train) likewise increase linearly during the linear increase in the size of the exposure aperture.
However, implementation of a truly stepless linear increase of pulse repetition frequency, can be rather problematic for reasons of complexity and cost.
An alternative to stepless adjustability of repetition frequency would be resort to a progressive stepwise increase of pulse-repetition frequency, i.e., by discrete successive increments. Nominally, the value of the scene-light signal to be totalized should exhibit a straight-line increase, i.e., appear as an inclined straight line when plotted on paper against elapsed time. Such an inclined straight line can, certainly, be approximated using a sufficiently great number of successive small stepwise increases, i.e., so that the difference between the staircase-like increase in the value (here, the repetition frequency) of the signal to be totalized, on the one hand, on the other hand, the inclined-straight-line increase ideally desired, not exhibit excessive percentage error, or what would appear to be excessive percentage error. However, the use of a large number of small increments in the repetition frequency to achieve such an approximation to straight-line increase can lead to considerable expense, because in most cases a discrete circuit stage would be needed for each respective one of the series of stepwise repetition-frequency increments.